1. Field of the Invention
The present invention relates generally to optical fibers, and particularly to the application of a coating to an exterior surface of the optical fiber.
2. Technical Background
Optical fibers have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This trend has had a significant impact in the local area networks (i.e., for fiber-to-home uses), which has seen a vast increase in the usage of optical fibers. Further increases in the use of optical fibers in local loop telephone and cable TV service are expected, as local fiber networks are established to deliver ever greater volumes of information in the form of data, audio, and video signals to residential and commercial users. In addition, use of optical fibers in home and commercial business for internal data, voice, and video communications has begun and is expected to increase.
The fibers used in local networks are directly exposed to harsh conditions, including severe temperature and humidity extremes. Since prior coatings for optical fibers did not perform well under such adverse conditions, the need existed for the development of higher performance coatings to address the wide and varied temperature and humidity conditions in which fibers are employed. Specifically, these coatings possessed thermal, oxidative, and hydrolytic stability which is sufficient to protect the encapsulated fiber over a long life-span (i.e., about twenty-five or more years).
Optical fibers typically contain a glass core, a cladding, and at least two coatings, i.e., a primary (or inner) coating and a secondary (or outer) coating. The primary coating is applied directly to the cladding and, when cured, forms a soft, elastic, and compliant material which encapsulates the glass fiber. The primary coating serves as a buffer to cushion and protect the glass fiber core when the fiber is bent, cabled, or spooled. Stresses placed upon the optical fiber during handling may induce microbending of the fibers and cause attenuation of the light which is intended to pass through them, resulting in inefficient signal transmission. The secondary coating is applied over the primary coating and functions as a tough, protective outer layer that prevents damage to the glass fiber during processing and use.
Certain characteristics are desirable for the primary coating, and others for the secondary coating. The modulus of the primary coating must be sufficiently low to cushion and protect the fiber by readily relieving stresses on the fiber, which can induce microbending and consequent inefficient signal transmission. This cushioning effect must be maintained throughout the fiber""s lifetime.
Because of differential thermal expansion properties between the primary and secondary coatings, the primary coating must also have a glass transition temperature (Tg) which is lower than the foreseeable lowest use temperature. This enables the primary coating to remain soft throughout the temperature range of use, facilitating differences in the coefficient of thermal expansion between the glass fiber and the secondary coating.
It is important for the primary coating to have a refractive index which is different (i.e., higher) than the refractive index of the cladding. This refractive index differential between the cladding and the primary coating allows errant light signals to be refracted away from the glass core.
Finally, the primary coating must maintain adequate adhesion to the glass fiber during thermal and hydrolytic aging, yet be strippable therefrom for splicing purposes. Moisture resistance is essential, because moisture also affects the adhesion of the primary coating to the glass. Poor adhesion can result in various sized delaminations which may lead to microbending and which can be significant sources of attenuation in the optical fiber.
Known adhesion promoters react to form bonds between the glass substrate and the primary coating or coating adjacent the substrate. Coatings which include these promoters are moisture sensitive and require the absence of water in the coating when the coating is in the liquid phase. Thus rigorous efforts are necessary to maintain the liquid coatings free of water.
Also the shelf life of these coatings is an issue. A previous attempt to increase the shelf life of the liquid coatings was to include a slow to hydrolyze poly(alkoxy)silanes in the coating. However, these coatings required unacceptable aging periods to allow adhesion to develop between the cured coating and the fiber.
One aspect of the present invention is a coating composition for siliceous surfaces. The coating includes at least one component from the group consisting of poly(alkoxy)silane, poly(halo)silane, alkoxysilane, halosilane, and mixtures thereof and a catalyst compound which generates a proton to hydrolyze the component when exposed to radiation. Preferably, the catalyst is a photo-acid.
In another aspect, the present invention includes a method of coating the optical fiber. The aforementioned coating is applied to an exterior surface of the fiber. A proton is generated to promote the hydrolysis of the component.
A preferred embodiment of the invention includes a method of accelerating adhesion between the exterior surface of the article and the coating. The inventive coating is applied to the surface of the fiber and the coating is exposed to a radiation source.
The coating of the invention has exhibited good adhesion properties with improved shelf life of the formulated liquid coating prior to application of the coating to the fiber. The invention also has the advantage that less reactive silanes may be used as an adhesion promoter. Also, the adhesion promoter of the invention is more hydrolyticly stable and has exhibited excellent adhesion properties in wet environments.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.